Conductive inks, obscuration enamels and decorative enamels typically use lead-based glass frits because they have a low melting range, low molten viscosity and stability against uncontrolled devitrification. Obscuration enamels are used in the automotive industry and conductive inks are used in the electronics industry, including in the manufacture of solar cells or photovoltaic cells.
Photovoltaic (“PV”) cells convert sunlight into electricity by promoting charge carriers in the valence band of a semi-conductor into the conduction band of the semiconductor. The interaction of photons in the sunlight and doped semiconductor materials form electron-hole pair charge carriers. These electron-hole pair charge carriers migrate in the electric field generated by the p-n semiconductor junction and collected by a conductive grid or metal contact printed or applied to the surface of the semiconductor, through which it flows to the external circuit. Crystalline silicon PV cells in today's industry are typically coated with an anti-reflective coating to promote light adsorption, which increases PV cells efficiency. However, the anti-reflective coating imposes high electrical resistance to the charge carrier flowing from the semiconductor to the metal contact. Such anti-reflective coatings often comprise silicon nitride, titanium oxide or silicon oxide.
Conductive inks are used to form these conductive grids or metal contacts. Conductive inks typically include a glass frit, a conductive species, such as silver particles, and an organic medium. To form the metal contacts, conductive inks are printed onto the substrate in a pattern of grid lines or other pattern by screen printing or other process. The substrate is then fired, during which electrical contact is made between the grid lines and the substrate. This contact is enhanced by the formation of individual silver crystallites at the glass-substrate interface. Without being bound by theory, it is believed that charge carriers are transferred from the substrate to the silver crystallites and then transferred to the gridline either through the glass layer by tunneling or directly to the silver of the gridline, if there is direct contact of the crystallite with both the gridline and the semiconductor. Lower firing temperatures are desirable in this process because of the lower cost involved and energy saved.
As otherwise mentioned herein, the anti-reflective coating enhances light absorption but also acts as an insulator which impairs the excited electrons from flowing from the substrate to the metal contacts. Accordingly, the conductive ink should penetrate the anti-reflective coating to form metal contacts having ohmic contact with the substrate. To accomplish this, conductive inks incorporate glass frits to aid with sintering silver particles to a substrate and to promote adhesion and ohmic contact between the formed metal contact and the substrate. When the glass frit liquefies, it tends to flow toward the interface between the silver particles and the anti-reflective coating on the substrate. The melted glass dissolves the anti-reflective coating materials as well as some of the metal particles and substrate. Once the temperature decreases, the molten silver and the melted or dissolved substrate recrystallize through the liquid phase. As a result, some of the silver crystallites are able to penetrate the antireflective layer and form ohmic contact with the substrate. This process is referred to as “fire-through” and facilitates a low contact resistance formation and a stronger bond between conductive grid or metal contact and the substrate.
The automotive, electronics and solar cell industries place greater emphasis on using environmentally-friendly components and processes. This emphasis has been further urged by the need to comply with environmental regulations. In response, the solar cell industry is moving to eliminate the use of lead in components and materials used in solar panels of cells.
Accordingly, there is a need for a lead-free glass frit which can be fired at a lower temperature and that can penetrate the anti-reflective layer and form metal contacts in ohmic contact with a substrate.